Control of selectivity is a central problem in catalysis, and this is particularly challenging with the microporous solid acid catalysts used in most industrial catalytic processes. Of particular interest here is methanol-to-olefin (MTO) catalysis, which has enormous commercial potential as discussed below.
Light olefins, defined herein as ethylene, propylene, and butylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily. For example, the large increase in demand for polyethylene has nearly exceeded the capacity of refineries to produce ethylene as a byproduct of petroleum conversion to gasoline.
Therefore, there is a need for alternative feedstocks for the production of light olefins. Amongst such feedstocks are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Consequently, because of the wide variety of Attorney Docket sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
Methanol is a particularly attractive olefin feedstock because it can be produced from natural gas, mostly methane, which is so inexpensive near the well head that it is frequently vented or flared. Converting natural gas to methanol has the added advantage of reducing pollution because venting or flaring natural gas causes emissions of methane and CO2, respectively, which are greenhouse gases. Further, methanol has the advantage that it, unlike natural gas, is easily transported using conventional tankers.
Because of the attractiveness of methanol as a potential olefin feedstock, there is intense developmental effort focused on converting methanol to olefins. Several major petrochemical companies are operating methanol-to-olefin (MTO) pilot plants, and large scale commercialization is expected in several years.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. For example, the earliest MTO processes were based on the aluminosilicate zeolite HZSM-5, a material with channels ca. 0.55 run in diameter. ZSM-5 has been extensively studied for the conversion of methanol to olefins. Unfortunately, ZSM-5 produces not only the desired light olefins, but also undesired by-products. In particular, ZSM-5 produces aromatics, particularly at high methanol conversion. Aromatics such as toluene and p-xylene readily diffuse through the ZSM-5 topology. At one time aromatics were used in gasoline fonnulations but arornatics are no longer desirable in US gasoline, and to serve as an MW catalyst, HZSM-5 must be modified to greatly reduce aromatic formation. This has been attempted, with some degree of success, by further restricting the channel size, usually by partial exchange with Mg2+ or by adsorbing an organic phosphorus compound and calcining to leave some kind of phosphate debris in the channel.
The foregoing points to the need for MTO catalysts that do not produce large amounts of unwanted by-products, such as aromatics, methane, carbon monoxide, and hydrogen gas. Furthermore, because ethylene and propylene are the most sought after products of the MTO conversion reaction, there is a need for catalysts that are most selective to ethylene and/or propylene, and for methods for increasing the selectivity of the reaction to ethylene and/or propylene.
Towards that end, zeolites with a small pore size are being developed because such zeolites have a higher selectivity to lower alkenes, even at 100 mol % methanol conversion. However, even though small pore size zeolites have a higher selectivity to lower alkenes, there is a need to further tailor zeolites to achieve other process objective. For example, amongst lower alkenes, there is a need for catalysts that selectively produce ethylene over propylene or vice versa.
This invention provides catalysts that provide such desirable characteristics, and also provides novel methods for tailor-making catalysts with desirable properties.